The present invention relates to heat pipes, and more particularly to a two-phase cold plate or heat absorbing panel constructed of an array of such heat pipes.
Two-phase cooling systems, particularly as embodied in heat pipes, have numerous advantages. Among these are near isothermality, very high heat transport capacity per unit of coolant moved (due to the phase change), low or zero energy required for coolant transport, and so forth. Disadvantages or conventional systems, however, have limited or prevented utilization in many circumstances. One such disadvantage is the relatively limited ability to transport meaningful coolant quantities over large distances. Another is the relatively limited ability to service many separate devices simultaneously, especially where the various thermal loads may be very different and/or significantly changing over time (e.g., equipment which is cycled on and off at various different times). As a result, single-phase cooling systems are still the system of choice in many configurations, such as the Space Shuttle, where two-phase loops could offer certain substantial advantages.
Pumped, two-phase heat-pipe style cooling loops (Thermal Bus) are currently being considered as candidates for overall thermal control of NASA's Space Station. As indicated, these systems transfer heat by evaporation and condensation rather than by sensible heat changes as in the conventional (e.g., Space Shuttle) single-phase cooling loops. Therefore, they operate at a substantially constant temperature over the entire length of the loop and are capable of transporting large thermal loads over long distances with relatively small pumping penalties. To be acceptable, however, such a system must perform independently of location. Also, specific heat exchangers ("sinks", or "cold plates") should be substantially isothermal, so that the temperature experienced by a piece of equipment which is mounted thereon will be substantially independent of its location on the plate. Also, the heat transfer between the equipment and the thermal bus fluid should occur with as low a temperature drop as possible, thus implying the need for high heat transfer coefficients. This is particularly important since the radiators represent the dominant weight of a space station's thermal control system. Finally, the plates should be able to transfer heat to the equipment so that it does not get too cold during periods when it is not operating. Thus, the system must be able to respond to substantially varying loads at a multiplicity of locations. At the same time, it should be uncomplicated, of minimum weight, reasonable cost, and high reliability.
Additionally, due to separate liquid and vapor lines, essentially no liquid should be allowed to exit the cold plates during normal operation. While some small amount of liquid is tolerable, larger amounts will lead to increased pressure drops in the vapor line caused by two-phase flow. Also, although the thermal bus is a mechanically pumped system, capillary forces distribute the coolant fluid in the heat pipe cold plate. Consequently, the plate performance in a gravity field may be somewhat lower than in 0-g. Thus, adequate heat flux capacity (e.g., 1W/cm.sup.2) should be provided in 1-g, to assure that 0-g performance will meet or exceed the system requirements. This implies a need to do more than merely control vapor pressure throughout the system, or furnish a constant coolant supply volume or pressure. It implies the need for active control of individual plate performance to assure a fully adequate supply of coolant therein, while also assuring that the plate is not flooded with too much.
A review of the prior art shows that heat pipe technology, of course, is well advanced. Monogroove heat pipes, in particular, are well developed and have many advantages for use in such a system. Also, the art includes many systems directed to means for controlling specific exchanger performance. It does not, however, appear to furnish the requirements just mentioned.
U.S. Pat. No. 4,515,207, issued May 7, 1985 (Alario et al.), for example, shows an excellent monogroove heat pipe with a bridging wick of spcific configuration. However, this patent does not disclose the use of active means for sensing the quantity of liquid in the liquid chamber and controlling a valve in response thereto.
U.S. Pat. No. 4,520,865, issued June 4, 1985 (Bizzell), U.S. Pat. No. 4,470,451, issued Sept. 11, 1984 (Alario et al.), and U.S. Pat. No. 4,422,501, issued Dec. 27, 1983 (Franklin et al.) show additional monogroove heat pipes.
U.S. Pat. No. 4,583,587, issued Apr. 22, 1986, shows a plurality of monogroove heat pipes welded together along their flanges.
U.S. Pat. No. 4,495,988, issued Jan. 29, 1985 (Grossman) discloses a means for sensing the vapor pressure in the vapor channel and then controls the pressure by means of a vapor pump.
U.S. Pat. Nos. 3,543,839, issued Dec. 1, 1970 (Shlosinger) 3,489,203, issued Jan. 13, 1970 (Fischell), and 3,414,050, issued Dec. 3, 1968 (Anand) disclose the use of valves in the vapor channel in order to control the heat conductivity of the pipes, in response to the temperature of the body whose temperature is being controlled by the heat pipes.
U.S. Pat. No. 4,308,912, issued Jan. 5, 1982 (Knecht) discloses the use of a valve in a solar heat collector for intermittently returning liquid from the condenser to the evaporator at periodic intervals.
U.S. Pat. Nos. 4,492,266, issued Jan. 8, 1985 (Bizzell et al.), 4,470,450, issued Sept. 11, 1984 (Bizzell et al.), 4,245,380, issued Jan. 20, 1981 (Maxson), 4,067,237, issued Jan. 10, 1978 (Arcella), 3,621,906, issued Nov. 23, 1971 (Leffert), and 3,517,730, issued June 30, 1970 (Wyatt) relate to heat pipes having various controlling means for altering the heat flow characteristics.
A need therefore remains for a two-phase cold plate having active control of individual plate performance to assure a fully adequate supply of coolant therein, while simultaneously assuring that the plate is not flooded with too much liquid. Further, such a plate must be able to respond to substantially varying loads at a multiplicity of locations. Additionally, it should be uncomplicated, of minimum weight, reasonable cost, and high reliability.